1. Field of the Invention
The present invention relates to an alternator driven by an internal combustion engine, for example, and relates to an automotive alternator mounted to an automotive vehicle such as a passenger car or a truck, for example.
2. Description of the Related Art
FIG. 20 is a cross section of a conventional automotive alternator, and FIG. 21 is a perspective of a stator in FIG. 20.
This alternator includes: a case 3 composed of an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 6 disposed within the case 3 having a pulley 4 secured to a first end thereof; a Lundell-type rotor 107 secured to the shaft 6; fans 105a and 105b secured to both axial end surfaces of the rotor 107; a stator 108 secured to an inner wall within the case 3; slip rings 9 secured to a second end of the shaft 6 for supplying electric current to the rotor 107; a pair of brushes 10 sliding on surfaces of the slip rings 9; brush holders 11 accommodating the brushes 10; rectifiers 12 electrically connected to the stator 108 for converting alternating current generated in the stator 108 into direct current; and a regulator 18 fitted over the brush holder 11 for adjusting the magnitude of the alternating voltage generated in the stator 108.
The rotor 107 includes a rotor coil 13 for generating magnetic flux on passage of electric current, and a pole core 14 disposed so as to cover the rotor coil 13, magnetic poles being produced in the pole core 14 by the magnetic flux. The pole core 14 includes a first pole core portion 121 and a second pole core portion 122 which intermesh with each other. The first pole core portion 121 and the second pole core portion 122 are made of iron and include disk portions 201 and 202 which are perpendicular to an axial direction, tapered claw-shaped magnetic poles 123 and 124 extending axially from the disk portions 201 and 202 in opposite directions to each other, and a cylindrical portion 200 connecting the disk portions 201 and 202 to each other, the circumference of the cylindrical portion 200 being covered by the rotor coil 13.
FIG. 22 is a perspective of the stator 108 in FIG. 20, FIG. 23 is a perspective of a stator core 115 in FIG. 22, and FIG. 24 is a partial plan of the stator core 115.
The stator 108 includes a stator core 115 for passage of a rotating magnetic field from the rotor coil 13, the stator core being formed by laminating a number of steel plates, and a stator winding 116 through which an output current flows. The stator core 115 includes an annular core back 82, a number of teeth 81 extending radially inwards from the core back 82 at an even pitch in a circumferential direction. The stator winding 116 is housed in a total of thirty-six slots 83 formed between adjacent teeth 81. The teeth include end portions 85 projecting in a circumferential direction of the stator 108, and post portions 86 connecting the end portions 85 to the core back 82. Spaces called opening portions 84 are formed between the end portions 85 of adjacent teeth 81.
In the automotive alternator of the above construction, electric current is supplied from a battery (not shown) through the brushes 10 and the slip rings 9 to the rotor coil 13, generating magnetic flux and giving rise to a magnetic field. At the same time, since the pulley 4 is driven by the engine and the rotor 107 is rotated by the shaft 6, a rotating magnetic field is applied to the stator core 115, generating electromotive force in the stator winding 116 and an output current is generated by an external load connected to the automotive alternator.
Now, the flux A generated by the rotor coil 13 leaves the first pole core portion 121, which is magnetized with north-seeking (N) poles, crosses an air gap between the rotor 107 and the stator 108, and enters the teeth 81 of the stator core 115. This magnetic flux A then passes through the core back 82, and flows from adjacent teeth across the air gap to the second pole core portion 122, which is magnetized with south-seeking (S) poles.
The amount of flux, which determines the output of the alternator, is itself determined by the magnetomotive force of the rotating magnetic field from the rotor 107 and magnetic resistance of the above magnetic circuit followed by the magnetic flux A. Consequently, if the magnetomotive force is constant, then it is important to shape this magnetic circuit to have the least resistance.
Furthermore, in order to improve the magnetomotive force, it is necessary to increase AT (the field current I multiplied by the number of turns n of conductor in the rotor coil 13), but AT is determined by installation space for the rotor coil 13 inside the pole core 114. When the overall size of the rotor 107 is limited, it becomes necessary to reduce the cross-sectional area of the magnetic path through the pole core 114 in exchange for increases in installation space for the rotor coil 13, and as a result the above-mentioned magnetic resistance increases, reducing the amount of magnetic flux passing through the pole core 114, and the magnetomotive force does not increase.
If one attempts to increase the magnetomotive force by increasing the field current I while keeping the cross-sectional area s of the conductor and the number of turns n constant, the temperature of the rotor coil 13 increases due to copper loss in the rotor coil 13, and the resistance of the conductor in the rotor coil 13 rises due to the increase in temperature, reducing the field current I, and the magnetomotive force does not increase after all.
On the other hand, as shown in FIG. 25, Japanese Patent Laid-Open No. HEI 11-164499 discloses an alternator aimed at increasing magnetomotive force by setting a ratio L1/L2 between an axial length L1 of the stator core 115 and an axial length L2 of the cylindrical portion 200 within a range of 1.25 to 1.75, placing the disk portions 201 and 202 opposite the stator core 115 so that the magnetic flux A flows directly from the disk portions 201 and 202 to the stator core 115, thereby increasing the cross-sectional area of the magnetic path through the pole core 114, and setting a ratio between an outside radius R1 of the claw-shaped magnetic poles 123 and 124 and an outside radius R2 of the cylindrical portion 200 between 0.54 and 0.60, thereby increasing the cross-sectional area of the magnetic path through the cylindrical portion 200.
However, in the case of this alternator, the figures are set with the aim of improving the output of the alternator per unit weight, and one problem has been that output decreases at low-speed rotation due to magnetic saturation.
Furthermore, by increasing the area of the disk portions 201 and 202 facing the stator core 115, thereby increasing the amount of overlap, the cross-sectional area of passages through valley portions 410 between the claw-shaped magnetic poles 123 and 124, which are passages for cooling ventilation, is reduced, increasing resistance to ventilation flow inside the rotor 107, and another problem has been that when a large field current I flows through the rotor coil 13, the cooling of the rotor coil 13 has been insufficient, increasing the resistance of the conductors in the rotor coil 13 and reducing the field current I, thereby preventing output from being increased.
The present invention aims to solve the above problems and an object of the present invention is to provide an alternator enabling the magnetic flux to be increased by increasing the cross-sectional area of the magnetic path, and also enabling output to be improved by reducing copper loss in the rotor coil.
To this end, according to the present invention, there is provided an alternator being such that a ratio (L2/L1) between an axial length L1 of disk portions and a length L2 of a stator core overlapping the disk portions in a radial direction is 0.3 or more, and a ratio (R2/R1) between an outside radius R1 of claw-shaped magnetic poles and an outside radius R2 of a cylindrical portion is within a range of 0.50 to 0.54.